Prompted by evidence suggesting that breast cancer patients treated in high-volume facilities have better long- term survival, the NY State Dept of Health enacted a mandatory regionalization policy whereby it would not reimburse low-volume facilities, defined as those with less than 30 all-payer breast cancer surgeries per year, for breast cancer surgeries provided to its beneficiaries. In this application, we take advantage of the natural experiment afforded by NY's Medicaid regionalization ruling (referred here as The Policy) to examine its impact on the distribution by volume, outcomes, and access to care for the approximately 15,000 NY residents undergoing surgery for an incident breast cancer every year. Specifically, our aims are: 1. To determine the extent to which The Policy led to Medicaid breast cancer patients, on average, being treated in higher volume facilities; 2. To evaluate the impact of The Policy on patient outcomes by examining the extent to which survival of NY Medicaid breast cancer patients improved relative to pre-Policy levels; and 3. To quantify unanticipated adverse consequences of The Policy by estimating its impact on access to treatment, and to simulate trade- offs between survival and access at different ?low-volume? thresholds. Using difference-in-difference-in- difference techniques, we contrast the experience of NY Medicaid breast cancer patients to historical and concurrent observational data on NY Medicare and commercially-insured breast cancer patients, as well as NJ Medicaid breast cancer patients, to estimate the effect of The Policy on outcomes, namely 5-year all-cause and breast-cancer-specific mortality and survival, and access measures, namely distance/time to treating facility and time from diagnosis to initial treatment, adjusting for patient's sociodemographic, tumor, and market characteristics. Analyses conducted with serial cohorts of patients with colorectal cancer will provide additional comparison groups. Despite increasing interest, the approach embodied in NY's Policy is not an uncontroversial proposition, so that adducing evidence of its impact would make a significant contribution to arguments for (or against) its broader diffusion. Given the large numbers of breast cancer patients, the evidence of consistent and persistent disparities in outcomes by the patients' socioeconomic status and hospital volume, and the unresolved questions concerning the challenges and value of regionalized breast cancer care, the results of this study will have significant implications for both policy and program development.